The invention provides methods for diagnosing biological states or conditions based on ratios of gene expression data from tissue samples, such as cancer tissue samples. The invention also provides sets of genes that are expressed differentially in malignant pleural mesothelioma. These sets of genes can be used to discriminate between normal and malignant tissues, and between classes of malignant tissues. Accordingly, diagnostic assays for classification of tumors, prediction of tumor outcome, selecting and monitoring treatment regimens and monitoring tumor progression/regression also are provided.